The present invention relates to a method of polishing a dental instrument and, more particularly, to a method of polishing a dental instrument such as a dental bracket, an artificial dental crown and a dental implant.
Conventionally, a dental instrument which is made of ceramics, such as an artificial dental crown, a dental implant, and a dental bracket generally has a complicated configuration with irregularities. The dental instrument is also small-sized.
Japanese Utility Model Laid-Open No. HEI 2-139617 discloses such a conventional dental bracket. The disclosed dental bracket is made of ceramics in place of a metallic material such as stainless steel or the like, for the purpose of an improvement in aesthetic appreciation. The disclosed dental bracket generally has a configuration as illustrated in FIGS. 8 and 9. The dental bracket 1 is arranged such that a pair of walls 3 and 4 forming a slot 2 is integrally provided continuously in unison over the entire widthwise length in a bracket body 5, and four (4) ligature-wire engaging wings 6.about.9 are integrally provided in extension or projection on both ends of an outer side surface of each of the slot-forming walls 3 and 4. A dimension of the dental bracket 1 per se is about 2.about.3.5 mm in width, about 2.about.3 mm in length, and about 2.about.3 mm in thickness. A groove width of the slot 2 is about 450.about.500 .mu.m and a depth thereof is about 700.about.900 .mu.m. As a material of the dental bracket 1, partially stabilized zirconia (e.g. zirconia stabilized with yttria), polymethylmethacrylate, dental composite resin or the like is used. The dental bracket 1 is integrally formed from these materials by injection molding. It is also formed by cast-molding a dental composite resin into a silicon rubber-made mold. Further, the dental bracket 1 may be prepared by subjecting these materials to grinding processing, although it is not so much preferable for the reason to be described later.
In use of the dental bracket 1 arranged as described above, a bottom surface of the bracket body 5 is bonded to and is fixedly mounted on a surface of a tooth; a long and narrow metallic wire (arch wire) 10 having a rectangular cross-sectional configuration passes through the slot 2; and the wire 10 is fixedly mounted on the slot 2 by another long and narrow wire (ligature wire) 11 which is engaged with the ligature-wire engaging wings 6.about.9. Under this condition, loads such as torsion, bending, tension and the like are applied to the arch wire 10, whereby the loads are transmitted or transferred to the tooth so that movement of the tooth occurs along the arch wire 10.
By the way, an oral cavity is placed under an insanitary environment due to various foods and drinks taken thereinto.
Since the oral cavity is under an insanitary environment as described above, dental plaque is liable to be deposited on the surface of the dental instrument, and various bacteria are liable to be bred or propagated. In case where the surface of the dental instrument is rough, plaque deposition and bacteria breeding are particularly remarkable. Furthermore, if the surface of the dental instrument is rough, there are problems that cracks are liable to be developed, and the strength is reduced. Therefore, it has been desired to solve such problems.
Moreover, the ceramic-made dental bracket 1 generally has a large frictional resistance which occurs between the inner surface of the slot 2 and the surface of the arch wire 10, as compared with a metallic bracket. For this reason, there is also a fundamental problem that an efficiency of moving the tooth is low and a time required for correcting the irregularities of the teeth is long.
Further, the arch wire 10 and the ligature wire 11 causes friction at their contact portions with the bracket 1. In the worst case, there is possibility that the wire 10 and the ligature wire 11 are cut off. These problems are related to a hardness and a surface roughness of the bracket material. In particular, since the ceramic-made bracket has high hardness, the wire and the like are worn at their portions which has been in contact with small irregularities of the dental bracket so that friction becomes larger.
If friction which occurs between the inner surface of the slot and the surface of the wire is so much large, there are problems that the efficiency of moving the tooth is lowered, and a period for the correction of the irregularities of the teeth is lengthened.
For the reason discussed above, it is required that the surface of the slot 2 is finished to a smooth surface with high accuracy, desirably, to a surface equal to or smaller than the maximum height R.sub.max =2 .mu.m in B0601 of JIS (Japanese Industrial Standard). In case where a dental bracket made of ceramics is produced, there are used a method in which a dental bracket product is formed by grinding a mother material of ceramics as described above, and a method in which a raw material powder is molded into a predetermined bracket configuration with a mold, and is sintered. In the former method, however, large-sized grinding defects occur. For this reason, even if polishing processing is carried out, it is difficult that the maximum height R.sub.max is controlled within an allowable limit. On the other hand, in the latter method, the defects in or on the surface of the mold into which the raw material powder is placed, are transferred to the surface of a dental instrument formed so that surface defects occur on the dental instrument. However, in the latter method, it is possible to maintain these defects smaller than the grinding defects in the former method. And by polishing the surface of the formed dental instrument, it is possible to further reduce the defects. As described previously, however, since the dental bracket of this kind is small-sized, and has also a complicated configuration, it is extremely difficult to polish the surface of the bracket, particularly, the inner surface of the slot.
Conventionally, as a method of polishing a surface of ceramics or the like, there is a polishing method using an acid or the like. However, there is a disadvantage that the polishing method using the acid can be applied to very limited ceramics such as alumina and the like.
Further, as disclosed in Japanese Patent Laid-Open No. HEI 1-177965, there is a method in which a material to be polished is barrel-polished in a mixture including a spherical resin, an abrasive and water. In the polishing method, a glass material or the like is polished with the resin and the abrasive whose grain size is small. Although the polishing method disclosed in the above Japanese patent gazette enables to polish a relatively plain or flat surface and a wide recess, it is not preferable that the disclosed polishing method is directly applied to polishing of the dental bracket 1. The reason for this is that a polishing speed is extremely slow, and clean polishing of an inner surface of a groove is impossible because the spherical resin is clogged in the slot 2 with extremely narrow groove width, within a short time. In view of this, further consideration has been made on this point. As a result, it has been found that the reason why the spherical resin is clogged in the groove is that the spherical resin per se is abraded or worn fastly because it has low hardness so that the configuration thereof becomes irregular. Moreover, it has been found that the reason why the polishing speed is low is due to the fact that, since the spherical resin has a smaller specific gravity than the ceramic-made dental bracket 1 to be polished, the spherical resin floats up to a surface of a polished liquid, and the polishing efficiency is lowered.